A principal problem encountered in this type of application is to be able to carry out measurements in a harsh environment, for which conventional sensor technologies cannot be used by reason of the harsh environmental conditions encountered, whether these be thermal, vibratory, chemical, etc. environmental conditions.
One approach used up to now to carry out such a measurement in such a difficult environment is to insert a transducer between the quantity to be measured and the measuring device, so that at the end of the transducer the environmental conditions are compatible with the technology of the sensor.
The function of the transducer is therefore to form a screen for some physical components that interfere with or are incompatible with the sensor, while being a transmitter of the quantity to be measured.
However, the use of this transducer presents a certain number of disadvantages, in particular by reason of deformations, loss or addition of data, etc. that this can produce.
These distortions are characterized for example by imperfections in amplitude (lack of precision or saturation) and/or in the dynamic domain (limited pass-band in low and/or high frequency).
An example of such a measurement in a harsh environment concerns the measurement of cylinder pressure in an internal combustion engine of a motor vehicle.
Measurement of such a cylinder pressure in an engine is a new requirement. It then consists of measuring, during various engine cycles (induction, compression, combustion, exhaust), the pressure variation of gases in order to optimize the control of injection and ignition in order to obtain better performance and therefore to reduce pollution.
However, the temperature varies during engine cycles from ambient temperature when the engine is at rest to several hundreds of degrees C., while being aware that the flame in the cylinder is at approximately 1800° C. Now, a certain number of materials used for sensors do not withstand such temperatures. This is for example the case with materials such as silicon that are normally used for such pressure measurements.
It has then been proposed to include the cylinder pressure sensor in a preheating plug, for example for diesel engines. The point of the plug is then situated at the top of the cylinder in contact with the flame. It is used as a transducer in order to compress, at the other end of the plug, a sensor that is located in a tempered environment in contact with the cylinder head of the engine cooled by water circulation for example.
However, transduction performed by the point of the preheating plug is subject to mechanical inertia (mass of the transducer) and, depending on the devices used, presents problems of leakproofness, as well as precision problems in low pressure values and/or it is sensitive to vibrations.
One variant of an embodiment of this assembly also consists of including the cylinder pressure sensor in the preheating plug, but differs from the preceding solution in that the fixing device gives support to the pressure transducer either side of the thickness of the cylinder head. Deformation of the cylinder head is then measured under the effect of pressure.
However, this measurement depends on the torque with which the preheating plug has been tightened in the cylinder head. Moreover, it detects vibrational modes of the cylinder head that are superimposed on the required signal. Finally, the measured signal is affected by the transfer function that depends on the cylinder head and its environment.
Another solution consists of using one or more optical fibers in order to detect the reflection of a light ray by a membrane subjected to pressure and temperature.
The optical fiber then conveys this data in a tempered location where electronics for acquiring and processing the signal are located.
However, the optical fiber also has temperature resistance limits. The membrane on which the light ray is reflected determines the quality of measurements. Its definition should take account of the mechanical characteristics (natural vibrational mode), and thermal characteristics (expansion deforming the membrane), problems of the mechanical aging of the membrane, problems of oxidation on its reflecting face, etc.
Moreover, the emergence of telecommunication technologies has been encountered, making it possible to communicate data and/or to supply power remotely to associated electronic devices.
Thus, for example, such means are used for measuring the pressure of a tire.
It is in fact difficult to lead electrical connections into a rotating part. A pressure sensor connected to the tire then experiences communication difficulties with the vehicle by reason of its movements in relation to this.
One solution consists of including a sensor with self-contained power supply inside the tire and a transmitter/receiver inside the vehicle. The sensor can then communicate by a radio frequency connection with the transmitter/receiver. In some devices, the sensor can also be remotely supplied by induction or by radio frequency.
Such a structure can also be used to provide temperature measurements in an oven.
In point of fact, it is difficult to lead electrical connections into an environment that is too hot, connections employing for example soldered joints being sensitive to this type of environment, in as much as for example such soldered joints can melt.
Use is also made in the automotive industry field of radio frequency means of communication for antitheft devices on vehicles.
Thus for example, an identification device can be incorporated into a key of the motor vehicle, this device not needing a supply in order to reduce maintenance to a minimum.
It is also possible to include a device, in the key, that, in response to a radio frequency signal, sends back a signal carrying data that is specific to the key. This device is then supplied by means of the power of the electromagnetic signal received.
Other examples using radio frequency means of communication for radio frequency identification of the RFID type employ passive or active chips combining power collection, processor, memory and radio frequency communication and are being developed on a large scale for identifications.